In 2005, he used his donor number to register with an online tracking site called the Donor Sibling Registry. Seisler learned he has at least 75 children, the story says. And the 33-year-old lawyer expects the number to grow to 120 to 140 children. He uses an Excel spreadsheet to keep track of them all.

Seisler is profiled in a Style network documentary that chronicled the issues, including the anger of the woman who became his wife. The program airs tonight. “What if they all come knocking?” she asks angrily on camera. “Did you think of the consequences that would come out of this?”

George Washington University law professor Naomi Cahn is among those calling for more regulation. Experts fear the explosion in half siblings could result in accidental incest or spread genes for rare diseases.